This invention relates to a micromechanical sensor.
Micromechanics is the technology whereby mechanical devices such as beams, springs and diaphragms are fabricated using techniques that are employed in the fabrication of monolithic integrated circuits. The fabrication of silicon micromechanical devices is described in J. B. Angell, S. C. Terry and P. W. Barth, "Silicon Micromechanical Devices", Scientific American, vol. 248, No. 4, pages 44-55 (1983). Micromechanical devices are typically made from silicon, but other materials such as gallium arsenide, certain ceramics and quartz have also been used.
A method of processing a silicon die so that a diaphragm of silicon is formed will be described with reference to FIG. 1. FIG. 1A shows a monocrystalline &lt;100&gt; silicon die 2 having opposite main surfaces 4 and 6. A layer 8 of silicon dioxide is formed over both surfaces 4 and 6 and is then removed from the surface 4, leaving the surface 6 covered. Boron is diffused into the die through the surface 4, forming a thin layer 10 adjacent that surface. A rectangular hole 12 (FIG. 1B) is formed in the oxide layer 8 using known photoprocessing operations, the sides of the hole being parallel to the &lt;110&gt; directions of the die. The exposed portion of the surface 6 is exposed to an anisotropic etchant, which forms a pit 14 in the silicon, having sloping sides 16 parallel to the {111} planes and a flat bottom 18. The etching proceeds until it reaches the doped layer 10, which acts as an etch stop (FIG. 1C). The portion of the layer 10 at the bottom of the pit thus forms a diaphragm 20.
It is known to form a cantilever beam of silicon by surface micromachining. For example, a layer of silicon dioxide may be formed on the front surface of a silicon die, and a layer of polysilicon deposited over the layer of silicon dioxide. The layer of polysilicon is patterned to define a strip that extends perpendicular to an edge of a larger area, and a layer of photoresist is deposited over the front surface of the processed die. An aperture is defined in the layer of photoresist, the aperture being sized and positioned so that the strip of polysilicon is exposed. The front surface of the structure is then exposed to an etchant that removes silicon dioxide but does not remove silicon, either in the polycrystalline or the monocrystalline form, and accordingly the silicon dioxide beneath the strip of polysilicon is removed. The photoresist is stripped. The resulting structure is a cantilever beam extending over an aperture in the layer of silicon dioxide.
Another method of making a cantilever beam by bulk micromachining is described in J. B. Angell, S. T. Terry and P. W. Barth, cited above.
J. Clarke, "SQUIDs, Brains and Gravity Waves", Physics Today, March, 1986, page 36, describes the superconducting quantum interference device, or SQUID, and explains that the SQUID can be used as a very sensitive magnetic flux detector.
There are two types of SQUID that have been developed, namely the two junction or dc SQUID and the single junction or rf SQUID. A dc SQUID is shown schematically in FIG. 2 and comprises a loop 30 of conductive material connected to two terminals 32 and 34 spaced apart around the loop, and two Josephson junctions 36 on the two sides of the loop. When the SQUID is biased by a constant current between the terminals 32 and 34, the voltage between the terminals is periodic in the magnetic flux threading the loop 30. The period of the variation in voltage is equal to the flux quantum, which is about 2 E(-15) Wb. The change in voltage can be observed with conventional test and measurement instruments, and therefore the dc SQUID can be used to measure very small changes in magnetic flux. It is known to fabricate dc SQUIDs using thin-film technology.